1. Field of the Invention
This invention relates generally to production of high purity deionized water and more specifically to the enhancement of deionized water by degasification at the point of use.
2. Description of the Prior Art
The requirement for high purity water with particular properties has evolved in several industries. The water purity requirements of the semiconductor industry are among the most demanding of any industry. While the yield of semiconductor chips is dependent upon a variety of factors, the average chip yield is directly related to the purity of deionized process water. As the size of the geometry used on the chips decreases, the adverse effects on yield of impurities in deionized (DI) process water increase. The requirement for high purity water in semiconductor processing is widely recognized. See for instance, U.S. Pat. No. 5,024,766 issued Jun. 18, 1991 to Shahzad Mahmud, incorporated herein by reference. See also U.S. Pat. No. 4,659,460 issued Apr. 21, 1987 to Muller et al., and U.S. Pat. No. 5,242,468 issued Sep. 7, 1993 to Clark et al. These other patent disclosures deal with various processes and apparatuses for purifying water.
For instance, U.S. Pat. No. 5,024,766 describes purifying DI water by lowering the total organic contaminant level of purified water from a plant purification system using an organic contaminant removal unit at the point of use. This describes further purification of water from a prior art plant water purification system, wherein the water purification unit is a small compact unit inserted between the water line from the purified deionized water line in the plant water purification system and the inlet of the point of use apparatus. The emphasis is on reducing organic contaminants.
U.S. Pat. No. 4,659,460 discloses a mobile fluid purification unit using demineralizers.
U.S. Pat. No. 5,242,468 is directed to ultra-high purity liquids for purposes of semiconductor fabrication. This is not limited to water but is also directed also to other liquids used in such fabrication. The treatment is performed at the point of use. The description of water filtration is directed to well known methods such as ion exchange and reverse osmosis in order to kill organisms, remove particles and organic matter and further removal of ions and sterilization.
As is well known, deionized water is used extensively in a typical semiconductor manufacturing processes. There are in typical semiconductor processes 100 to 200 steps in which DI water is applied to semiconductor wafers. DI water is applied to the wafers in 25 to 50 steps of such processes typically where an aluminum alloy (metallization) is present on the wafers. Deionized water is used in the wafer fabrication and assembly processes because of the low level of contaminants present. As is well known and as described in the prior art, contaminants in xe2x80x9ctapxe2x80x9d water such as sodium, potassium, nitrates, organics, bacteria, and others contribute to the failure of microelectronic devices. A measure of the quality of DI water is resistivity. The higher the resistivity, the lower the level of the ionic contaminants and the higher the quality. As integrated circuit dimensions continue to be reduced, greater demands are placed on the purity of DI water. The response of the semiconductor industry is to continue to drive up the requirements for the resistivity of DI water i.e., its purity. For instance, an American Society for Testing Materials (ASTM) requirement for electronic grade type E-1 (point of use ultra-pure water) in U.S. Pat. No. 5,024,766 is a resistivity of 18 megaohms-cm. This was in 1988. Current semiconductor fabrication techniques require greater purity in terms of e.g. particle filtration, and reduction in dissolved silica.
Thus a common practice in DI water manufacturing is the injection of air into the deionized water to remove carbon dioxide which is a contaminant from the reverse osmosis step. This saturates the deionized water with oxygen to a typical oxygen concentration of 8,500 ppb (parts for billion) at 25xc2x0 C. Such oxygen has typically not been considered a contaminant or particularly problematic in the prior art.
However, it is also well known that DI water in the prior art etches aluminum and aluminum alloys (the metallization on semiconductor wafers), creating undesirable pits and voids in the metal leading to device failures, thereby reducing yield and/or reduction in long term reliability of the fabricated circuits.
Thus semiconductor manufacturers are forced to go to great lengths (automation, dry-to-dry processing, critical queue times and other techniques) to ensure that semiconductor wafers with metallization (aluminum) on them do not remain in contact with DI water for extended times. It is also recognized that the etch rate of aluminum is undesirably accelerated by increasing the temperature of the DI water. It is known also that it is desirable to use high temperature DI process water because of its improved solvating efficiency. Thus in the prior art use of high temperature DI water is precluded by the increased undesirable aluminum etch rate.
Therefore, it is generally recognized that there are problems in the use of DI water and especially high temperature DI water during semiconductor processing steps where aluminum or aluminum alloys are present on the wafers. However, it is not believed that any prior art solutions have been proposed.
The present inventor has recognized that the problematic reaction between DI water and aluminum alloys is not due to the water itself but to the dissolved oxygen present in the DI water. It is not believed that the exact nature of the drawbacks of DI water were recognized in the prior art; instead, DI water in general was believed to be problematic when in contact with aluminum and aluminum alloys. This prior art conclusion was erroneous as determined by the present inventor.
The present inventor instead has found that DI water is problematic when used in contact with aluminum or aluminum alloys only due to the oxygen dissolved in DI water, due to the prior step of injection of air to remove the undesirable carbon dioxide or due to other sources of air or oxygen coming into contact with the DI water. The supplied deionized water has a concentration of dissolved oxygen gas exceeding 3000 ppb. Therefore, the present inventor has found that degasification of DI water immediately before it reaches the metallized wafers overcomes the problems with aluminum processing. The degasification process removes the dissolved oxygen which has been identified by the present inventor as the oxidizing agent that leads to the undesirable etching or pitting of the aluminum. Thus it has been found by the present inventor that DI water without oxygen does not etch aluminum or aluminum alloys. No pits or voids are produced in the aluminum portions of a semiconductor wafer that has been rinsed with degasified DI water.
Removing the dissolved oxygen thus allows the use of higher temperature DI water on metallized wafers and longer rinsed times. This increases yield and improves circuit reliability. The degasification also eliminates the inconvenience of necessarily removing the wafers immediately from e.g., a DI rinse tank after a rinse cycle is complete. This offers increased flexibility in semiconductor processing by relaxing one critical process parameter.
While the presently disclosed embodiment is in the context of semiconductor processing, this is illustrative and not limiting.